The present invention relates generally to the installation, repair, and preventive maintenance of asphalt paved surfaces. More particularly the present invention relates to a hot-applied binder composition for use in tack coat applications, stress absorbing membrane interlayers (SAMI) and non-tracking stress absorbing membrane interlayers (NT-SAMI) for asphalt paving and road construction. More specifically, the present invention relates to a non-tracking, hot-applied binder application with improved low temperature flexibility for use beneath asphalt mix courses.
In the installment of asphalt paving mix courses for the purpose of building, repairing or maintaining existing roads, the application of an adhesive material is required beneath the mix courses to provide a bond to the existing, underlying surface. This application of adhesive material is also required between mix courses when constructing new roads.
Inadequate adhesion beneath or between asphalt mix courses can often lead to premature cracking, delamination or slippage beneath and/or between mix courses and the overall failure of roadways. The results of premature failure can be dangerous for vehicular traffic and generally are costly to correct for responsible agencies and taxpayers.
Adhesive materials for bonding asphalt mix courses are known in the art and are referred to as tack coats. A typical tack coat may be comprised of a certain percentage of asphalt cement liquid, emulsifiers, water or other additives and may be applied at hot, warm or ambient climate temperatures.
A common phenomenon associated with asphaltic emulsions (comprised of asphalt cement, water and emulsifier) when used in tack coat compositions is that a certain amount of curing time is required to release the water phase of the emulsions. Application rates are usually in the range of 0.05 to 0.15 gallon per square yard of about 60% asphalt content emulsion. Generally, curing times run from the start of the tack coat application to the beginning of asphalt mix course installation. The minimum curing time is determined by the speed that water is released from the asphalt emulsion material. Higher application rates take longer to cure and create traction issues for the construction equipment. During the curing time which can run from 15 minutes to hours, it is often the case that the traveling public is not permitted to drive on the tack coat, resulting in costs related to roadway lane closures or detouring of traffic. Ironically, during this curing period it is a very common industry practice for asphalt paving equipment and asphalt mix hauling trucks to drive on the tack coat application. The results of this common practice often lead to the tack coat being displaced, picked up or tracked away by the haul vehicle tires or tracks from its intended area of adhesion. As a result this practice can lead to incomplete bonding in the areas damaged by construction traffic which corresponds often with the wheel paths of traffic later.
Tack coats may be comprised of liquid asphalt cement and are typically applied at elevated temperatures and are generally referred to as “hot-applied tack coats”. Typical application rates of hot-applied tack coats are about 0.05 to 0.1 gallon per square yard. Paving grade asphalt cements have been used, with a range of different consistencies, from softer grades, such as 150 penetration or higher, to harder grades with penetrations below 100. While these materials only require the temperature to drop to typically below 140° F. prior to the beginning of paving, they often are still prone to picking up on paving equipment tires or tracks. An inadequate amount of tack coat material, prior to the placement of the hot mix asphalt paving course, results in premature pavement failures, such as described above in reference to the use of asphalt emulsion tack coat materials.
In an effort to overcome tracking issues associated with asphalt emulsion tack coats or hot-applied tack coats, a category classified as “non-tracking tack coat” materials have been developed. The non-tracking tack coat materials have been implemented in the form of asphalt emulsion and hot-applied asphalt materials. While application rates may be similar to other hot-applied tack coats, due to the harder asphalt used in the formulation, higher application rates may be used, reported to be as high as 0.2 gallon per square yard. Hot-applied, non-tracking tack coats are reported to cure faster than conventional hot-applied tack coats and asphalt emulsion tack coats. Hot-applied, non-tracking tack coats are typically comprised of asphalt liquid cements having a penetration (pen) of less than 40 dmm. These low pen materials result in brittle behavior for their intended use in tack coat applications and as a result of the brittleness lead to bond failure demonstrated by premature longitudinal wheel path cracking and delamination of asphaltic mix courses.
An additional issue with emulsified tack coats is that generally if they are applied at too high of an application rate they run (flow) before curing due to the low viscosity of the emulsions. When paving on hills or curves with grades this problem of running (flowing) often is dealt with by reducing the application rate. While highway agencies want a higher rate applied, it is difficult with to achieve higher rates with emulsions which represent at least 95% of all tack coats used today. Further contractors do not like the mess of the emulsion tack coats on their equipment so they often apply the lightest rate that they can without the highway agencies stopping them. In most cases, if left to the contractors, no tack coat would be applied.
Stress absorbing membrane interlayers (SAMI) are typically asphalt binders applied, either hot or in an emulsion format, over an existing distressed pavement, most commonly asphalt concrete pavements. The application rates are greater than typically used in tack coat applications, typically 0.4 gallon per square yard or higher for an emulsion based product. The asphalt binders are polymer modified to provide greater elasticity with the ability to resist underlying pavement stresses from transmitting into the new asphalt overlay. After application of the SAMI binder, aggregate is placed over the binder to prevent construction equipment or vehicular traffic from picking up the binder. The textured surface created by the embedded aggregate in the SAMI provides a mechanical interlock with the new asphalt overlay. While the interlock aids in the ability to compact the new overlay, the uncoated aggregate surface lacks an actual adhesive bond to the new overlay. The lack of an adhesive bond between the SAMI and the new overlay may result in longitudinal wheel path cracking.